Lithium ion batteries currently used in the fields of electrical energy storing stations and electromobiles are generally formed of a plurality of individual batteries having large capacities that are connected in series. Ideally, the individual batteries in a battery pack have the same operation voltage and the same actual capacity as each other. However, for an actual battery pack, it is difficult to ensure that all the individual batteries have the same operation voltage and the same actual capacity as each other even before shipment of the battery pack, and as the number of charging and discharging operations of the battery pack increases, the inconsistency among the individual batteries in the battery pack will become greater. The inconsistency among the individual batteries will compromise operational performances such as the charging efficiency and the discharging capability of the battery pack, and meanwhile, will shorten the service life of the battery pack.
To solve the aforesaid problem, a practice in the prior art is to decrease the inconsistency among the individual batteries in the battery pack by means of a parallel resistor balancing method. The parallel resistor balancing method is to connect a resistor balancing circuit in parallel with each of the individual batteries so that excessive energy is consumed by the resistor. Although this practice uses only a simple circuit structure, it can only balance a small current, the heat generation is significant and the heat dissipating cost is high; and meanwhile, the balance can only be performed at the end of a charging period, so the electricity amount that can be balanced is limited. Further, this method can only accomplish the balance among individual batteries managed by a single battery management control unit (BMU).
Another practice in the prior art is to decrease the inconsistency among the individual batteries in the battery pack by means of an inductor balancing method. The inductor balancing method is to use an inductor as an energy storing element to transfer electrical energy from the individual batteries. Also, this practice can only achieve the balance among adjacent individual batteries managed by a single BMU, and the balancing time is long.
Therefore, in the process of balancing among the individual batteries in the battery pack, the prior art practices can only achieve the balance among individual batteries managed by a single BMU but cannot achieve the balance among all the individual batteries of the whole battery pack.